1. Field of the Invention
The present invention relates to a distortion compensation apparatus for use in a transmitter that transmits signals.
2. Description of the Related Art
In recent years, mobile communication is commonly used. Effective utilization of communication resources (especially, radio frequency) has been highly demanded. In response to such demands, a wireless system using a linear modulation system (such as QPSK) that uses frequencies efficiently has been practically used.
The wireless system is often required to be small in size and to be operated with low electric power consumption. The wireless system using the linear modulation method requires a transmit circuit having a high linearity. However, a high output power amplifier (a power amplifier) is generally disposed at a last stage of the transmit circuit in the wireless system. Therefore, it is difficult to meet these requirements at the same time. That is to say, when the signal is amplified at a linear region of the high output power amplifier, the linearity is good, but the efficiency of the amplifier becomes poor. In this case, electric power consumption becomes high, which requires a large power source or makes heat dissipation conditions severe. On the other hand, when the signal is amplified in a non-linear region in the high output power amplifier, the efficiency of the amplifier becomes high, but a good linearity cannot be provided. When the linearity becomes poor, the signal is greatly distorted. The characteristic of signal of an adjacent channel becomes poor due to leak power and the like.
To solve the above-mentioned problems, a distortion compensation apparatus is used. The wireless system including the distortion compensation apparatus amplifies the signal in the non-linear region in the high output power amplifier to decrease the electric power consumption, and the distortion of the signal produced in the amplifier is compensated in the distortion compensation apparatus.
FIG. 1 is an illustration for explaining a basic operation of a distortion compensation apparatus. The distortion compensation apparatus is in a digital pre-distortion type in this explanation. It is assumed that an input signal X (t) is processed at a processing unit 1, and is outputted as a signal Y (t).
The distortion compensation apparatus detects or estimates the distortion produced at the processing unit 1, produces a compensation signal that compensates the distortion, and provides the compensation signal to the signal X (t). Specifically, a subtraction circuit 11 produces an error signal 23 representing a difference between the reference signal 21 and the feedback signal 22. The reference signal 21 is not distorted, and is obtained by simply delaying the signal X (t) for a predetermined time. On the other hand, the feedback signal 22 is distorted at the processing unit 1. Accordingly, the error signal 23 corresponds to the distortion produced at the processing unit 1.
A distortion compensation unit 12 comprises a table for storing information that produces a compensation signal 24. A distortion compensation signal updating unit 13 updates the table in the distortion compensation unit 12 based on the error signal 23. The distortion compensation unit 12 produces the compensation signal 24 based on the information extracted from the table using an amplitude or a power of the signal X (t) and the like as a search key. The compensation signal 24 cancels the error signal 23.
A multiplier 14 multiplies the compensation signal 24 to the input signal X (t). Since the distortion produced at the processing unit 1 is compensated by the compensation signal 24, the signal Y (t) has no distortion (or suppressed distortion).
In the distortion compensation apparatus, in order to compensate the distortion adequately, a correct error signal 23 should be produced. In order to provide the correct error signal 23, the timing between the reference signal 21 and the feedback signal 22 should be matched accurately. Accordingly, as shown in FIG. 2, a digital pre-distortion type distortion compensation apparatus generally comprises a delay unit 31 and a delay quantity control unit 32.
The delay unit 31 delays the feedback signal. The delay quantity in the delay unit 31 follows instructions from the delay quantity control unit 32. The delay quantity control unit 32 calculates the delay quantity so that the timing between the reference signal 21 and the feedback signal 22 delayed in the delay unit 31 is matched, and notifies the result to the delay unit 31. As a method of adjusting the timing between the signals (i.e., a method of determining the delay quantity of the feedback signal), it is known to utilize the correlation between the above-mentioned signals, or ACLR (Adjacent Channel Leakage Ratio).
However, due to the trouble or the like, if the reference signal 21 or the feedback signal 22 does not reach the subtraction circuit 11, or if the reliability of the signals is decreased, the erroneous delay quantity is calculated at the delay quantity control unit 32. Once the erroneous delay quantity is determined at the delay quantity control unit 32, an incorrect error signal 24 is produced based on the erroneous delay quantity. As a result, the distortion cannot be compensated. In some cases, the feedback system may be run away and the distortion may rather be increased. When such a situation occurs, the signal of an adjacent to channel may be adversely affected. Accordingly, it is desirable that the distortion compensation apparatus having the configuration described above have a protection function so that the delay quantity calculated in the delay quantity control unit 32 does not indicate an abnormal value.